Hydraulic cylinder arrangements typically include a piston member coupled to a rod member to form a piston and rod assembly. Various coupling arrangements have been used in the past to couple a piston member to a rod member.
For example, in a standard piston and rod assembly, a rod member is provided having a threaded reduced-diameter end. A piston member is provided to be received over the rod member end and for slideable engagement with a non-threaded region adjacent to the rod member end. A piston nut can be threadably engaged with the rod member end to retain the piston member on the rod member.
In another example, a piston and rod assembly 5 is shown in FIG. 1. Here, the piston member 6 itself is threaded onto the rod end 7 using standard M-class threads having a root radius of about 0.125 pitch, or when the pitch is 3, a root radius of 0.375 mm. Further, to prevent relative rotation between the piston member and the rod member, a pair of offsetting sinuLOC™ setscrews are used through the body of the piston member to engage the rod end. Although the assembly in FIG. 1 performs satisfactory for lighter duties, such as low pressure, low duty cycle, and/or low life cycle applications, such as found in application requiring backhoe loaders, there remains a need for an assembly that can handle heavier duties.
Another piston and rod assembly is described in Korea Pat. App. Publ. No. KR 20100094186A. The publication discloses a piston and rod assembly including a rod having an externally threaded end mated to an internally threaded bore of a piston. To prevent loosening of the piston from the rod, a piston nut is also threadably engaged to the rod end and in contact with the piston. One or more setscrews are inserted through corresponding radial openings extending radially through the inner circumference of the piston nut to engage the rod end. For instance, two setscrews are disposed off center with respect to the center of the rod. Further, the position of the stepped interface between the piston and the rod closer to the head end than the rod end undesirably increases the risk for stress riser development at that interface and thus joint failure.
As described previously, prior piston and rod assemblies may rely substantially upon engagement between a slidably engaged rod and piston joint with a nut threaded to the rod end for coupling a piston member and a rod member together. However, for such configurations, high torque, such as, e.g., 13,000 to 20,000 Nm, is used to tighten the nut to the rod end and form the retention joint between the piston and rod assemblies. As a result of the high torque application, the threads are tension pre-loaded and typical only the first three threads between the nut and rod end withstand a majority of the load, leaving the remaining threads underutilized. Hence, application of axial loading from both rod end and head end directions in combination with pre-load provides a total greater stress range to the threads, leaving the threads highly susceptible to fatigue and loosening. To couple the nut to the rod end, the rod end is often machined to a reduced effective cross-sectional area, thereby leaving a smaller cross-sectional area to withstand the higher loads. Consequently, the rod end must be processed in a manner for higher strength (such as constructed of higher strength materials and/or heat treated), thereby increasing the overall costs of production and/or increasing the risk of stress risers. Further, the fatigue of the threaded joint joined by high torque is substantially reduced compared to lower torque applications. Therefore, reducing or eliminating such reliance may facilitate stronger or otherwise more practical or economical structural configurations than provided with prior coupling arrangements.
The embodiments described herein are directed at overcoming one or more of the disadvantages associated with prior piston and rod assemblies.